The present invention relates generally to disc drives. More particularly, the present invention relates to reducing resonant oscillation of the disc drive mechanical structure.
A typical disc drive includes one or more discs mounted for rotation on a hub or spindle. A typical disc drive also includes one or more transducers supported by a hydrodynamic air bearing, which flies above each disc. The transducers and the hydrodynamic air bearing are collectively referred to as a data head. A drive controller is conventionally used for controlling the disc drive system based on commands received from a host system. The drive controller controls a disc drive to retrieve information from the discs and to store information on the discs.
An actuator operates within a negative feedback, closed-loop servo system. The actuator moves the data head radially over the disc surface for track seek operations and holds the transducer directly over a track on the disc surface for track following operations. A servo controller samples the position of the data heads relative to some reference point and generates an error signal based upon the difference between the actual position and the reference position. This error signal is then used to drive the data head to the desired reference point, typically by demanding a current through a voice coil motor (VCM) which forms part of the actuator.
Information is typically stored on the discs by providing a write signal to the data head to encode flux reversals on the surface of the disc representing the data to be stored. In retrieving data from the disc, the drive controller controls the actuator so that the data head flies above the disc, sensing the flux reversals on the disc, and generating a read signal based on those flux reversals. The read signal is then decoded by the drive controller to recover the data represented by flux reversals stored on the disc, and consequently represented in the read signal provided by the data head.
Thus, a disc drive mechanical structure is composed of multiple mechanical components that are pieced together to form the final disc drive assembly. Each of these components has various resonant modes that if excited by an external energy source will cause the part to physically move at the natural frequencies of oscillation for the component in question. This movement can occur in a bending mode, a twisting mode or a combination of the two. If the component is highly undamped (i.e. the resonance is high amplitude, narrow frequency band) it will tend to oscillate with a minimal external driving energy. This oscillation results in physical motion of the data head, causing off track errors and potential fly height problems. These oscillations are often referred to as xe2x80x9cresonances.xe2x80x9d
If resonances occur in a disc drive, they can severely limit drive performance, both in seek mode and track-follow mode. To obtain the optimal disc drive performance requires that there be no resonances present. However, this scenario is not physically possible. Every mechanical component has a natural frequency of oscillation. Nevertheless, it is desirable to reduce or minimize the resonances. One way of doing this is to mechanically damp the mechanical components and thereby decrease the amplitude of the resonant mode. This can be done by careful design, the end result being a reduction in the amplitude of the oscillation to a level that is deemed acceptable to achieve a desired drive performance.
However, there are situations where a component is not able to be mechanically damped. This could occur, for example, because of materials used or because of design time constraints. When this scenario occurs, the only way to improve drive performance is to make sure that no excitation energy at the natural frequency of oscillation reaches the mechanical component to start it oscillating. The present invention concentrates on this approach.
As mentioned above, typical disc drives demand a current through a voice coil motor (VCM) to drive the data head to the desired position. When a frequency spectrum of demand current is analyzed it is found that the spectrum is composed of frequency components from direct current (DC) all the way up to multiple kilohertz (kHz). If VCM current is driving the actuator at the same frequency as the natural frequency of a mechanical resonant mode of a mechanical component, the energy may be sufficient to excite the mechanical structure into oscillation. This is very undesirable and will at least degrade disc drive performance or at worst will cause the servo system to go unstable.
The method employed by servo engineers to minimize the chances of the mechanics oscillating is to use hardware electronic filtering and/or digital filtering of the VCM current via a microprocessor or digital signal processor. Both types of filters achieve the same overall result. They reduce the driving force energy (i.e. the current flowing) at frequencies deemed a concern.
One type of filter that is widely used to remove driving energy at the mechanical resonant modes is known as a notch filter. A notch filter is a band-rejection filter that produces a sharp notch in the frequency response curve of the disc drive. When a notch filter is activated by the servo control loop, the open loop response becomes a summation of the original response plus the notch filter response. If the notch filter is centered about the frequency where the peak amplitude of the mechanical resonance occurs, then the driving force energy at this frequency can be reduced so that there will be little or no energy made available to excite the mechanical structure.
The problem with the notch filter, however, is that if the center frequency of the mechanical resonance does not align with the center frequency of the notch filter then the attenuation of the driving current may not be enough to keep the structure from going into oscillation. This will occur if the mechanical resonance has shifted in frequency. This can easily occur on a drive to drive basis or even from one data head to another.
Present disc drives have fixed notch filters that are designed to cover a spread in mechanics. Such a filter, for example, is described in U.S. Pat. No. 5,032,776. Such filters remove driving energy at frequencies which would not cause the mechanical structure to oscillate for a given head or for a given drive. Thus, they are not optimal solutions. Furthermore, such filters cannot guarantee that the gain of the resonance will remain below 0 dB.
Methods also exist to implement adaptive filtering techniques by implementing digital signal processing algorithms in the servo controller. Such a method, for example, is described in U.S. Pat. No. 5,325,247. Such methods involve complex microcontroller code and are heavy on computational time. Furthermore, such methods cannot also guarantee optimal results under all circumstances.
As disc drive servo systems continually require higher open loop bandwidths to track follow accurately, the requirement for improved filtering techniques increases also. The present invention provides an economical means of providing a high degree of attenuation of the mechanical resonance frequencies and offers other advantages over the prior art.
The present invention relates to a method and apparatus for providing improved attenuation of the mechanical resonant frequencies in a disc drive.
One embodiment of the invention is directed to a method of filtering the actuator driving energy to reduce the frequency components which correspond to resonant frequencies that are present in the disc drive mechanical structure using a notch filter. In accordance with this embodiment, a resonant frequency range is determined by establishing a minimum resonant mode and a maximum resonant mode by performing a bode/structural plot of the disc drive. When the resonant frequency range is less than a coverage limit, the notch filter is used to attenuate the range of resonant frequencies. However, if the range of resonant frequencies exceeds the coverage limit, then the disc drive can be failed.
Another embodiment of the present invention is directed to a computer disc drive having various resonant modes covering a resonant frequency range. The disc drive includes at least one disc, multiple data heads, an actuator, a servo control processor and a digital notch filter. The discs are capable of storing data. The data heads are capable of reading data from and writing data to the discs. The actuator is coupled to the data heads for positioning the data heads relative to the discs in response to driving energy. The servo control processor is coupled to the actuator and is capable of providing driving energy to the actuator. The digital notch filter is adapted to attenuate the frequency components of the actuator driving energy contained within the resonant frequency range of the disc drive. Notch filter constants which define the frequency response of the digital notch filter are stored in a servo flash memory of the servo control processor. The servo control processor loads the notch filter constants when the disc drive is used.